Field
This application relates generally to high voltage semiconductor devices and methods of making the devices and, in particular, to high voltage semiconductor devices comprising an integrated diode and methods of making the devices.
Background of the Technology
Metal oxide semiconductor field-effect transistors (i.e., MOSFETs) are commonly used in power electronic circuits such as DC to DC converters. DC-DC converters use power MOSFET based switches to convert voltage from one level to another level. In a typical DC to DC converter, a control circuit drives the gates of two power MOSFETs to regulate the transfer of power from the supply to a load. One of the power MOSFETs may be operated as a synchronous rectifier.
The properties of silicon carbide are ideally suited for high-voltage power electronic applications such as power MOSFETs for DC-DC converters. One of the main advantages of silicon carbide over silicon is its higher critical breakdown field strength. Silicon carbide has breakdown field strength of approximately 3 MV/cm compared to approximately 0.3 MV/cm for silicon. The 10× higher breakdown field strength of silicon carbide enables semiconductor switches and rectifiers with higher reverse blocking voltages and lower on state resistance enabling superior power electronic system performance than possible with silicon.
In a DC-DC converter, the power MOSFETs need to be turned off for a short period of time while one MOSFET is turning on and the other is turning off to prevent shoot through current between supply and ground. During this dead time, the p-n junction diode integral to the power MOSFET structure can conduct current. Current conduction through the p-n junction diodes integral to SiC MOSFETs is not preferred due to higher conduction power losses compared to Schottky diodes. The higher conduction power loss of SiC p-n junction diodes is the result of a higher turn on voltage and therefore a larger forward voltage drop of the diodes. P-n junction diodes also have a higher switching power loss since it is a bipolar device and stores minority carriers that need to be removed for the diode to turn off.
For this reason, a Schottky diode can be connected anti-parallel with the SiC power MOSFET as a freewheeling diode (D1) in the DC-DC converter circuit. The Schottky diode has a lower turn on voltage (approximately 0.9 V) and therefore has a lower conduction loss compared to the integral p-n junction diode with a forward voltage drop of approximately 3.5V. Also, a Schottky diode is a majority carrier devices, so the switching power losses are also lower compared to p-n junction diodes since there is no storage of minority carriers in the device.
While freewheeling Schottky diodes can be added to the converter circuit to improve the conversion efficiency, the use of an external Schottky diode increases the cost of the converter unit due to the need for an additional component. External Schottky diodes also take up room on the board which hinders achieving a smaller converter footprint. The reliability of the Schottky diode and its electrical connections on the board also may reduce the overall reliability of the converter. In addition, the wire-bonds in the Schottky diode result in additional inductance which will play a role in limiting the high frequency operation of the converter.
Accordingly, there still exists a need for MOSFET devices, particularly SiC MOSFET devices, wherein a Schottky diode is integrated within the power MOSFET structure.